This invention relates to image display systems, and more particularly to an optical system for an SLM-based display system.
Spatial light modulators (SLMs) have found application in many fields, a significant one of which is image displays. In general, an SLM is an array of light-emitting, light-transmitting, or light-reflecting elements, which are individually addressable, usually with electronic signals. Many SLMs are binary, having an addressing scheme that switches its elements to either an xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d state to form the image. A characteristic of SLMs is that there is no scanningxe2x80x94all pixels are activated at substantially the same time to generate the entire image or a two-dimensional block of the image, depending on the size of the image and the SLM.
One type of SLM is a digital micro-mirror device (DMD), also known as the digital light processor (DLP), manufactured by Texas Instruments Incorporated. The DMD has an array of thousands of tiny tilting mirrors. To permit the mirrors to tilt, each is attached to one or more hinges mounted on support posts and each is spaced by means of an air gap over underlying addressing circuitry. The addressing circuitry provides electrostatic forces, which cause each mirror to selectively tilt.
For display applications, the DMD is addressed with image data. In accordance with this image data, light is selectively reflected or not reflected from each mirror and projected onto a viewing screen. The combination of light and dark mirrors forms an image. Modulation techniques are used to provide greyscale image xe2x80x9cframesxe2x80x9d. A quick succession of frames is perceived by the viewer as a full motion display.
There are at least two approaches to generating color displays with the DMD display system. One approach is to generate multiple images with multiple SLMs, typically one SLM each for red, green and blue. Each image has a desired intensity, and the images are combined to result in the correctly colored display. A second approach is to use a single SLM and generate images for each color (red, green, and blue) sequentially. A white light source is filtered through a revolving color wheel, such that a desired color illuminates the corresponding image. The differently colored images are generated so quickly that the eye integrates them into the correctly colored frame.
For SLM-based projection systems, there are two basic architectures for the optical system from the light source to the image plane. Each of these architectures has distinct advantages and disadvantages, depending on the type of display system.
Non-telecentric architectures demonstrate superior contrast as compared to telecentric architectures. This is due to the higher illumination angles at the SLM. However, non-telecentric architectures also produce higher projection angles to the screen. This is due to the high amount of lens offset required to separate illumination and projection paths without using a prism or other element. These high angles are not problematic for front screen projection system, but do cause problems for rear-screen projection systems, which need to fold the image into a small cabinet space.
Telecentric architectures do not require offset, and can therefore greatly reduce lens field and screen angle problems. This makes them preferable for rear screen projection systems. However, telecentric architectures have inherently lower contrast due to lower illumination angles on the SLM.
One aspect of the invention is an optical engine for use in an SLM-based display system. A light source and illumination optics provide high angle illumination to the SLM. The light reflected from the SLM enters a relay path having a first set of lenses, a mirror, and a second set of lenses. The first set of lenses receives the SLM output in an offset pupil. The mirror reflects the light to the second set of lenses, which creates the image at an intermediate image plane. At this point, the image is telecentric and on-axis. A projection lens magnifies the image from the intermediate image plane to a display image plane.
An advantage of the invention is that it combines characteristics of both telecentric and non-telecentric designs. It provides the best features of each, while avoiding their shortcomings.
Like a non telecentric design, the optical path provides a display having optimum contrast. A higher illumination angle is accomplished at the SLM, with the resulting offset being compensated by the relay path.
At the same time, the optical path permits the projection lens to be telecentric and therefor to be smaller than would be the case for a non telecentric design. The offset and working distance of the projection lens can be minimal for the particular application. For different applications, variations to configuration of the projection lens may be easily accomplished, in that the projection lens is easily accessible and can be readily switched with other configuration lenses.
The same optical engine may be used for both front and rear screen projection systems. For front screen systems, the image created by the relay path may be bound by a mask placed within the optical path at a location that is easily accessed. This eliminates the need for a mask on the SLM itself, making the SLM device less expensive and easier to manufacture. This mask in the image plane has the same effect as a cabinet bezel in a rear projection display system, thus enabling front-screen projection with no boundary artifacts outside the active picture area. The high contrast provided by the high-angle optical engine also eliminates the need for special contrast enhancing coatings on a DMD type SLM, as well as preserves black level uniformity due to telecentricity at the SLM.